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JavaScript
JavaScript, often abbreviated as JS, is a , that conforms to the specification. JavaScript has , , , and s. Alongside and , JavaScript is one of the core technologies of the . JavaScript enables interactive s and is an essential part of s. The vast majority of s use it, and major s have a dedicated to execute it. As a multi-paradigm language, JavaScript supports , , and (including and ) s. It has for working with text, , dates, s, and the , but the language itself does not include any , such as , , or facilities. It relies upon the host environment in which it is embedded to provide these features. Initially only implemented in web browsers, JavaScript engines are now embedded in many other types of host software, including in web servers and databases, and in non-web programs such as word processors and software, and in runtime environments that make JavaScript available for writing mobile and desktop applications, including desktop widgets. The terms Vanilla JavaScript and Vanilla JS refer to JavaScript not extended by any frameworks or additional libraries. Scripts written in Vanilla JS are plain JavaScript code. Although there are similarities between JavaScript and , including language name, , and respective , the two languages are distinct and differ greatly in design. JavaScript was influenced by programming languages such as and . The format, used to store s in or transmit them across , is based on JavaScript. History Beginnings at Netscape In 1993, the (NCSA), a unit of the , released , the first popular graphical , which played an important part in expanding the growth of the nascent beyond the niche where the had formed three years earlier. In 1994, a company called was founded in and employed many of the original NCSA Mosaic authors to create . However, it intentionally shared no code with NCSA Mosaic. The internal codename for the company's browser was , a portmanteau of "Mosaic and Godzilla". The first version of the Web browser, Mosaic Netscape 0.9, was released in late 1994. Within four months it had already taken three-quarters of the browser market and became the main web browser for the 1990s. To avoid trademark ownership problems with the NCSA, the browser was subsequently renamed Netscape Navigator in the same year, and the company took the name Netscape Communications. Netscape Communications realized that the Web needed to become more dynamic. , the founder of the company, believed that needed a "glue language" that was easy to use by Web designers and part-time programmers to assemble components such as images and plugins, where the code could be written directly in the Web page markup. In 1995, Netscape Communications recruited with the goal of embedding the programming language into its Netscape Navigator. Before he could get started, Netscape Communications collaborated with to include Sun's more static programming language, , in Netscape Navigator so as to compete with for user adoption of Web technologies and platforms. Netscape Communications then decided that the scripting language they wanted to create would complement Java and should have a similar syntax, which excluded adopting other languages such as , , , or Scheme. To defend the idea of JavaScript against competing proposals, the company needed a prototype. Eich wrote one in 10 days, in May 1995. Although it was developed under the name Mocha, the language was officially called LiveScript when it first shipped in beta releases of Netscape Navigator 2.0 in September 1995, but it was renamed JavaScript when it was deployed in the Netscape Navigator 2.0 beta 3 in December. The final choice of name caused confusion, giving the impression that the language was a spin-off of the Java programming language, and the choice has been characterized as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new Web programming language. There is a common misconception that JavaScript was influenced by an earlier Web page scripting language developed by named Cmm (not to be confused with the later created in 1997). Brendan Eich, however, had never heard of Cmm before he created LiveScript. Nombas did pitch their embedded Web page scripting to Netscape, though Web page scripting was not a new concept, as shown by the Web browser. Nombas later switched to offering JavaScript instead of Cmm in their ScriptEase product and was part of the TC39 group that standardized ECMAScript. Server-side JavaScript In December 1995, soon after releasing JavaScript for browsers, Netscape introduced an implementation of the language for with . Since 1996, the web-server has supported Microsoft's implementation of server-side Javascript— —in ASP and .NET pages. Since the mid-2000s, additional implementations have been introduced, such as in 2009. Adoption by Microsoft Microsoft script technologies including and were released in 1996. JScript, a implementation of Netscape's JavaScript, was part of . JScript was also available for server-side scripting in . Internet Explorer 3 also included Microsoft's first support for CSS and various extensions to HTML, but in each case the implementation was noticeably different from that found in Netscape Navigator at the time. These differences made it difficult for designers and programmers to make a single website work well in both browsers, leading to the use of "best viewed in Netscape" and "best viewed in Internet Explorer" logos that characterized these early years of the . JavaScript began to acquire a reputation for being one of the roadblocks to a cross-platform and standards-driven Web. Some developers took on the difficult task of trying to make their sites work in both major browsers, but many could not afford the time. With the release of , Microsoft introduced the concept of , but the differences in language implementations and the different and proprietary s remained and were obstacles to widespread take-up of JavaScript on the Web. Standardization In November 1996, Netscape submitted JavaScript to to carve out a standard specification, which other browser vendors could then implement based on the work done at Netscape. This led to the official release of the language specification published in the first edition of the ECMA-262 standard in June 1997, with JavaScript being the most well known of the implementations. and were other well-known implementations of . The release of ECMAScript 2 in June 1998 continued the standards process cycle, conforming some modifications to the ISO/IEC 16262 international standard. ECMAScript 3 was released in December 1999 and is the modern-day baseline for JavaScript. The original ECMAScript 4 work led by Waldemar Horwat (then at Netscape, now at ) started in 2000. initially participated and implemented some proposals in their language. Over time it was clear that Microsoft had no intention of cooperating or implementing proper JavaScript in Internet Explorer, even though they had no competing proposal and they had a partial (and diverged at this point) implementation on the server side. So by 2003, the original ECMAScript 4 work was mothballed. The next major event was in 2005, with two major happenings in JavaScript's history. First, Brendan Eich and Mozilla rejoined Ecma International as a not-for-profit member and work started on (E4X), the ECMA-357 standard, which came from ex-Microsoft employees at (originally acquired as Crossgain). This led to working jointly with (later acquired by ), who were implementing E4X in ActionScript 3 (ActionScript 3 was a fork of original ECMAScript 4). So, along with Macromedia, work restarted on ECMAScript 4 with the goal of standardizing what was in ActionScript 3. To this end, Adobe Systems released the ActionScript Virtual Machine 2, code named , as an project. But Tamarin and ActionScript 3 were too different from web JavaScript to converge, as was realized by the parties in 2007 and 2008. Alas, there was still turmoil between the various players; —then at —joined forces with Microsoft in 2007 to oppose ECMAScript 4, which led to the ECMAScript 3.1 effort. The development of ECMAScript 4 was never completed, but that work influenced subsequent versions. While all of this was happening, the open source and developer communities set to work to revolutionize what could be done with JavaScript. This community effort was sparked in 2005 when released a white paper in which he coined the term , and described a set of technologies, of which JavaScript was the backbone, used to create web applications where data can be loaded in the background, avoiding the need for full page reloads and leading to more dynamic applications. This resulted in a renaissance period of JavaScript usage spearheaded by open source libraries and the communities that formed around them, with libraries such as , , , , and others being released. In July 2008, the disparate parties on either side came together in Oslo. This led to the eventual agreement in early 2009 to rename ECMAScript 3.1 to ECMAScript 5 and drive the language forward using an agenda that is known as Harmony. ECMAScript 5 was finally released in December 2009. In June 2011, ECMAScript 5.1 was released to fully align with the third edition of the ISO/IEC 16262 international standard. ECMAScript 2015 was released in June 2015. ECMAScript 2016 was released in June 2016. The current version is ECMAScript 2017, released in June 2017. Later developments JavaScript has become one of the most popular programming languages on the Web. However, many professional programmers initially denigrated the language due to the perceived target audience of Web authors and other such "amateurs". The advent of returned JavaScript to the spotlight and brought more professional programming attention. The result was a proliferation of comprehensive , improved JavaScript programming practices, and increased usage of JavaScript outside Web browsers, as seen by the proliferation of platforms. In January 2009, the project was founded with the goal of specifying a common standard library mainly for JavaScript development outside the browser. With the rise of s and JavaScript-heavy sites, it is increasingly being used as a compile target for s from both and . Trademark "JavaScript" is a of in the United States. It is used under license for technology invented and implemented by Netscape Communications and current entities such as the . Features The following features are common to all conforming ECMAScript implementations, unless explicitly specified otherwise. Universal support All popular modern Web browsers support JavaScript with built-in interpreters. Imperative and structured JavaScript supports much of the syntax from (e.g., if statements, while loops, switch statements, do while loops, etc.). One partial exception is : JavaScript originally had only with var. ECMAScript 2015 added keywords let and const for block scoping, meaning JavaScript now has both function and block scoping. Like C, JavaScript makes a distinction between and . One syntactic difference from C is , which allows the semicolons that would normally terminate statements to be omitted. Dynamic ; Typing:JavaScript is like most other s. A is associated with a rather than an expression. For example, a initially bound to a number may be reassigned to a . JavaScript supports various ways to test the type of objects, including . ; Run-time evaluation: JavaScript includes an function that can execute statements provided as strings at run-time. Prototype-based (object-oriented) JavaScript is almost entirely . In JavaScript, an is an , augmented with a prototype (see below); each string key provides the name for an object property, and there are two syntactical ways to specify such a name: dot notation (obj.x = 10) and bracket notation (obj'x' = 10). A property may be added, rebound, or deleted at run-time. Most properties of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using a for...in loop. JavaScript has a small number of built-in objects, including Function and Date. ; Prototypes: JavaScript uses where many other object-oriented languages use for . It is possible to simulate many class-based features with prototypes in JavaScript. ; Functions as object constructors: Functions double as object constructors, along with their typical role. Prefixing a function call with new will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the Object prototype). ECMAScript 5 offers the Object.create method, allowing explicit creation of an instance without automatically inheriting from the Object prototype (older environments can assign the prototype to null). The constructor's prototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the function used as a constructor. JavaScript's built-in constructors, such as Array or Object, also have prototypes that can be modified. While it is possible to modify the Object prototype, it is generally considered bad practice because most objects in JavaScript will inherit methods and properties from the Object prototype, and they may not expect the prototype to be modified. ; Functions as methods: Unlike many object-oriented languages, there is no distinction between a function definition and a definition. Rather, the distinction occurs during function calling; when a function is called as a method of an object, the function's local this keyword is bound to that object for that invocation. Functional A is ; a function is considered to be an object. As such, a function may have properties and methods, such as .call() and .bind(). A nested function is a function defined within another function. It is created each time the outer function is invoked. In addition, each nested function forms a : The of the outer function (including any constant, local variable, or argument value) becomes part of the internal state of each inner function object, even after execution of the outer function concludes. JavaScript also supports s. Delegative JavaScript supports implicit and explicit . ; Functions as roles (Traits and Mixins): JavaScript natively supports various function-based implementations of patterns like and s. Such a function defines additional behavior by at least one method bound to the this keyword within its function body. A Role then has to be delegated explicitly via call or apply to objects that need to feature additional behavior that is not shared via the prototype chain. ; Object composition and inheritance: Whereas explicit function-based delegation does cover in JavaScript, implicit delegation already happens every time the prototype chain is walked in order to, e.g., find a method that might be related to but is not directly owned by an object. Once the method is found it gets called within this object's context. Thus in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions. Miscellaneous ; Run-time environment:JavaScript typically relies on a run-time environment (e.g., a ) to provide objects and methods by which scripts can interact with the environment (e.g., a webpage ). It also relies on the run-time environment to provide the ability to include/import scripts (e.g., Compatibility considerations Because JavaScript runs in widely varying environments, an important part of testing and debugging is to test and verify that the JavaScript works across multiple browsers. The DOM interfaces are officially defined by the in a standardization effort separate from JavaScript. The implementation of these DOM interfaces differ between web browsers. JavaScript authors can deal with these differences by writing standards-compliant code that can be executed correctly by most browsers. Failing that, they can write code that behaves differently in the absence of certain browser features. Authors may also find it practical to detect what browser is running, as two browsers may implement the same feature with differing behavior. Libraries and toolkits that take browser differences into account are also useful to programmers. Furthermore, scripts may not work for some users. For example, a user may: * use an old or rare browser with incomplete or unusual DOM support; * use a or browser that cannot execute JavaScript; * have JavaScript execution disabled as a security precaution; * use a speech browser due to, for example, a visual disability. To support these users, Web authors can try to create pages that on user agents (browsers) that do not support the page's JavaScript. In particular, the page should remain usable albeit without the extra features that the JavaScript would have added. Some sites use the HTML tag, which contains alt content if JS is disabled. An alternative approach that many find preferable is to first author content using basic technologies that work in all browsers, then enhance the content for users that have JavaScript enabled. This is known as . Security JavaScript and the DOM provide the potential for malicious authors to deliver scripts to run on a client computer via the Web. Browser authors minimize this risk using two restrictions. First, scripts run in a in which they can only perform Web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by the : scripts from one Web site do not have access to information such as usernames, passwords, or cookies sent to another site. Most JavaScript-related security bugs are breaches of either the same origin policy or the sandbox. There are subsets of general JavaScript—ADsafe, Secure ECMAScript (SES)—that provide greater levels of security, especially on code created by third parties (such as advertisements). is another project for safe embedding and isolation of third-party JavaScript and HTML. is the main intended method of ensuring that only trusted code is executed on a Web page. Cross-site vulnerabilities A common JavaScript-related security problem is (XSS), a violation of the . XSS vulnerabilities occur when an attacker is able to cause a target Web site, such as an online banking website, to include a malicious script in the webpage presented to a victim. The script in this example can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim's authorization. A solution to XSS vulnerabilities is to use HTML escaping whenever displaying untrusted data. Some browsers include partial protection against reflected XSS attacks, in which the attacker provides a URL including malicious script. However, even users of those browsers are vulnerable to other XSS attacks, such as those where the malicious code is stored in a database. Only correct design of Web applications on the server side can fully prevent XSS. XSS vulnerabilities can also occur because of implementation mistakes by browser authors. Another cross-site vulnerability is (CSRF). In CSRF, code on an attacker's site tricks the victim's browser into taking actions the user did not intend at a target site (like transferring money at a bank). When target sites rely solely on cookies for request authentication, requests originating from code on the attacker's site can carry the same valid login credentials of the initiating user. In general, the solution to CSRF is to require an authentication value in a hidden form field, and not only in the cookies, to authenticate any request that might have lasting effects. Checking the HTTP Referrer header can also help. "JavaScript hijacking" is a type of CSRF attack in which a